1. Field of the Invention
The present invention relates to a plasma CVD apparatus used for forming a thin film on a substrate such as a semiconductor substrate, and a method for forming a thin film and a method for forming a semiconductor device using the plasma CVD apparatus.
2. Description of the Related Art
A plasma CVD apparatus is used for forming a thin film on a substrate, as in the case of forming a semiconductor device such as a thin film transistor (TFT) on a semiconductor substrate. Such a plasma apparatus is disclosed in, for example, Japanese Laid-Open Publication No. 2002-270600.
FIG. 10 is a schematic cross-sectional view showing a structure of a conventional plasma CVD apparatus 400. The plasma CVD apparatus 400 includes a plate-like cathode electrode 1 disposed in an upper portion of a container 10 in a horizontal position and a plate-like anode electrode 2 disposed in a lower portion of the container 10 in a horizontal position. The cathode electrode 1 and the anode electrode 2 are disposed in parallel to each other with a predetermined spacing therebetween. A substrate holder 3 is disposed between two electrodes. A substrate 4 is placed on and fixed to a surface (an upper surface) of the substrate holder 3.
The cathode electrode 1 in the upper portion of the container 10 may also serve as a “shower plate”, i.e., a uniform ventilation distribution plate for uniformly supplying a raw material gas. The anode electrode 2 in the lower portion of the container 10 is connected to ground. A heater is integrally provided with the anode electrode 2 to heat the substrate 4 supported by the substrate holder 3.
The substrate holder 3 is supported in a movable manner in a horizontal direction by rollers 5 provided on the anode electrode 2. By rotating the rollers 5, the substrate holder 3 which supports the substrate 4 is moved in a horizontal direction with respect to a region 8 between the cathode electrode 1 and the anode electrode 2.
An exhaust system 6 is provided in the container 10. Inside the container 10 is put under a high vacuum by the exhaust system 6. A raw material gas is supplied into the container 10 under a high vacuum. The raw material gas to be supplied to the container 10 is supplied between the cathode electrode 1 and the anode electrode 2 by the cathode electrode 1 which serve as a shower plate. An RF power supply 7 applies a high-frequency voltage between the cathode electrode 1 and the anode electrode 2. Thus, plasma discharge occurs in the region 8 between the cathode electrode 1 and the anode electrode 2. Plasma formed in the region 8 decomposes the raw material gas. Thus, a thin film is formed on a surface of the substrate 4 disposed below the region 8.
In the plasma CVD apparatus 400 shown in FIG. 10, the electrode in the lower portion of the container 10 is connected to ground. Thus, the electrode in the lower portion of the container 10 is used as the anode electrode 2 and the electrode in the upper portion of the container 10 is used as the cathode electrode 1. Alternatively, the cathode electrode 1 in the upper portion of the container 10 may be connected to ground. Furthermore, the substrate holder 3 is close to the anode electrode 2 in the lower portion of the container 10. However, it may be close to the cathode electrode 1 in the upper portion of the container 10.
Japanese Laid-Open Publication No. 4-293782 discloses a substrate holder used for a plasma CVD apparatus for producing a thin film on a substrate.
In the small-size plasma CVD apparatus 400 having a structure as shown in FIG. 10, the substrate holder 3 which is movable in a horizontal direction is provided on the anode electrode 2 integrally formed with the heater. The substrate 4 is fixed to and supported by the substrate holder 3. Thus, in-line transfer for transferring the substrate holder 3 to which the substrate 4 is fixed into the container 10 is possible.
When the substrate holder as described in Japanese Laid-Open Publication No. 2002-270600 is used, the substrate holder is transferred into the container by in-line transfer.
However, in a plasma CVD apparatus in which the substrate holder is movable and in-line transfer is possible, a space 9 is formed between the substrate holder 3 and the anode electrode 2. This causes a problem that, when a voltage is applied between the cathode electrode 1 and the anode electrode 2, an unnecessary discharge occurs in the space 9.
For example, in the plasma CVD apparatus 400 shown in FIG. 10, when the rollers 5 provided on the anode electrode 2 are electrically connected to the substrate holder 3, discharge concentrates to portions around the rollers 5. Thus, uneven discharge is not formed on the surface of the substrate 4 which is fixed to the substrate holder 3 and in portions near the rollers 5.
In order to prevent occurrence of unnecessary discharge concentrated in portions around the rollers 5, it is considered to make the substrate holder itself serve as an anode electrode. Without providing another anode electrode, the substrate holder itself is made to have an electrode function and is connected to ground. Thus, no space is formed below the substrate holder, which serves as an anode electrode, thereby preventing occurrence of unnecessary discharge.
However, in such a structure, in order to make the substrate holder movable in a horizontal direction for allowing the in-line transfer, only a limited portion of the substrate holder is connected to ground. Thus, the substrate holder may not be sufficiently grounded. If the substrate holder is not sufficiently grounded entirely, plasma discharge is not uniformly formed between the substrate holder and the cathode electrode. As a result, uneven discharge such as that formed in the plasma CVD apparatus 400 shown in FIG. 10 may be undesirably generated on the substrate fixed to the substrate holder.
Furthermore, not only in a structure where the substrate holder is made movable, but also in a structure where the anode electrode and the heater are separately provided to make the heater movable, a space is formed between the substrate and the anode electrode. Thus, there is a problem that unnecessary discharge occurs in such a space.